Valves are used in numerous subsea applications. In one application, they are used to regulate the amount of chemical inhibitors injected into the production stream flowing from a subterranean formation to the surface. Chemicals such as methanol, for example, are used to inhibit the formation of hydrates in the tubing used to carry the production from the wellhead to the platform. Other chemicals such as corrosion inhibitors, scale inhibitors, and wax inhibitors, for example, may also be injected into the production stream at or near the wellhead. Typically, such valves are located either at or near the subsea wellhead and/or at or near a subsea manifold.
In another application, subsea valves are used to control the flow of oil, natural gas and other wellbore fluids from the subterranean surface. In a subsea production environment, there are various tree configurations that incorporate such subsea valve assemblies. In the typical subsea arrangement, valves of this type are incorporated into the tree piping. This arrangement typically requires the entire tree to be removed in order to retrieve and/or service such control valves.
Occasionally, it becomes necessary to vary the amount of chemical inhibitors being injected into the production stream and/or vary the flow of product being transmitted from the wellhead to the rig. This has previously not been possible with a fixed orifice device. Rather, variable flow orifice devices have been used for this purpose. One example of such a system employs electronic controls to vary the adjustment of the orifice device. A drawback of such systems, however, is that where multiple subsea wellheads and manifolds are employed throughout the ocean floor very often miles of electrical cable are needed to network the plurality of valves employed in such systems. This has proven to be a very expensive and not very reliable means of varying the control of chemical inhibitors being injected into the production stream.
Other systems have employed variable choke constrictions in subsea applications that are adjusted using remotely operated vehicle (ROV) devices. A drawback of such devices is that it is difficult to determine whether the remotely operated vehicle (ROV) has adjusted the choke to the desired position and such systems are very expensive and complex.